1. Field of the Invention
The present invention relates to a thermal head, and more particularly to a wide-spanning thermal head constructed by combining a plurality of head substrates.
2. Description of the Prior Art
Thermal printers are used as printing devices for various information processing apparatus. In recent years, wide-spanning thermal heads have come to be employed for thermal printers to provide enlarged printing areas. When, for example, printing is to be made on a JIS (Japanese Industrial Standard) A2 size recording paper with its longer side as the printing direction, a wide-spanning thermal head is required which can provide a printing width of about 600 mm. However, it is difficult to arrange fine heating resistance elements in a straight line array along the length of about 600 mm on a single head substrate made for example of ceramics while maintaining uniform heating characteristics along the entire length. Therefore, a total printing width of 600 mm is usually realized, for example, by combining two head substrates each having numerous heating resistance elements arranged along its horizontal length of about 300 mm, the opposing ends thereof along the arranged direction being abutted against each other.
FIG. 1 is a perspective view showing the structure of a prior art thermal head 1 of such construction. The thermal head 1 comprises head substrates 3a and 3b which are made of ceramics such as aluminum oxide Al.sub.2 O.sub.3 and each formed in a rectangular plate shape and on which numerous heating resistance elements 2 formed for example from tantalum nitride Ta.sub.2 N or the like are arranged in a straight line array. Radiator plates 4a and 4b made of aluminum and formed in a rectangular plate shape are bonded to the head substrates 4a and 4b, respectively, using an elastic adhesive 5. This allows slight displacement of the head substrates 3a and 3b relative to the radiator plates 4a and 4b. The radiator plates 4a and 4b are fixed to a support plate 6 made of aluminum or other metallic material. The thus formed heating resistance elements 2 are selectively energized and heated to achieve printing by heating on a thermosensitive paper, for example.
The spacing .delta.1 between the heating resistance elements 2 on the head substrates 3a and 3b is, for example, 15 .mu.m, with a pitch of 8 dots/mm. On the other hand, the spacing .delta.2 between the heating resistance elements 2 adjacent to each other across the junction between the two head substrates 3a and 3b needs to be set at less than twice the spacing .delta.1 in order to avoid the so-called blanking which results in the occurrence of a white streak where no image is produced when thermal printing is performed. Therefore, in the prior art construction, the distance from the heating resistance elements 2 adjacent to each other across the junction between the head substrates 3a and 3b to the respective ends of the head substrates 3a and 3b is appropriately determined so as to provide a prescribed spacing therebetween, while the head substrates 3a and 3b are secured to the radiator plates 4a and 4b in such a way that the opposing end faces 7a and 7b of the head substrates 3a and 3b become flush with the opposing end faces 8a and 8b of the radiator plates 4a and 4b, with the end faces 7a and 8a abutting against the end faces 7b and 8b respectively.